Linear approximate conservation laws were already proposed as
a tool for model reduction of CRNs when the so-called quasi-equilibrium (QE) condition [1]}, [2]} is satisfied. At QE the direct and reverse rates of fast reversible reactions compensate each other and the net rates of change of reactants and products are negligible. Products or reactants of fast reactions are fast species. However, although
concentrations of fast species are equilibrated, these variables can not be eliminated by using the quasi-steady state (QSS)
equations. In the case of QE linear combinations of concentrations
of fast species are conserved by the fast dynamics and the QSS equations have degenerate solutions indexed by the values of the conserved quantities [1]}, [2]}.
